Gauging Wind Direction is Worth a Second Look

Gauging Wind Direction is Worth a Second Look

NASA's
ER-2 high-altitude research aircraft carried SSL's test model of a new sensor
for measuring winds at sea level. The ER-2 is seen being readied for flight
(left) and as it climbs out above the old blimp hangars at Ames Research
Center (right).
Click on image for the whole picture.

March 26, 1997: A new way of looking at the
"body heat" of the world's oceans may let satellites measure wind
direction at sea level by combining microwave images of the ocean taken
from two different angles.

Project scientist Robbie Hood of the Space Sciences Laboratory at Marshall
Space Flight Center said the effect is based on what many boaters already
know: wind-driven waves look different if viewed upwind versus downwind.

Measuring wind direction will help in understanding how the Earth's climate
system works, and contribute to improved weather forecasts. Principal Investigator
for this project is Dr. Roy Spencer, a scientist at NASA/Marshall. Both
work at the Global Hydrology and Climate Center, a NASA-university partnership.

"While there are existing satellites that measure wind speed and
direction at the ocean surface," Hood said, "they use radars that
cost ten times as much, and their spatial coverage is not as extensive,
as the passive microwave radiometers we are testing for this application".

An object does not have to be red hot to glow. Everything emits electromagnetic
energy at all wavelengths. The human body emits infrared radiation that
can be detected by special cameras, for example.

The NASA scientists are measuring microwave emissions by the ocean surface
at a frequency of 37 gigahertz (37 GHz), about 300 times higher than the
FM radio band. For years weather satellites have carried microwave sounding
units that measure the microwave emissions of oxygen at 60 GHz as an aid
to measuring air temperature. The microwave signals are measured at different
polarizations, similar to the use of polarized sunglasses to limit glare
off of a water surface.

Listening to the surf

If the ocean is very smooth, it emits less microwave energy. As it roughens
with wave action, more energy is emitted by small waves near the wavelength
of the radiation (0.8 cm) and by foam generated by the wave action. This
provides the basis for the measurement of wind speed.

But which way is the wind blowing?

Engineers at Marshall built the C-STAR, the Conically-Scanning Two-look
Airborne Radiometer for flight tests aboard NASA's ER-2 high-altitude research
jet.

C-STAR is shown (left) mounted upside down for checkout on the ground
and (right) in the belly of the ER-2 aircraft. The twin mirrors are mounted
at the end of a short boom. The circular objects at the top (right) are
the antennas. The white object is a calibration heat source.
Click on image for the whole picture.

"With C-STAR, we hope to measure how the wind is deforming the ocean
surface and from that measure the wind speed and direction," Hood said.

The idea is simple. Foam tends to collect on the lee side of waves. Looking
at the same piece of ocean from different angles, and comparing the two
signals, should tell which direction the wind is blowing from.

A different angle on the problem

Getting the different angles was done by the "conically scanning"
aspect of C-STAR. The radiometers were put on a turntable that spins once
every 10 seconds to look ahead and then behind of the ER-2 aircraft. The
aft-looking scan covers ocean seen 4 minutes earlier by a forward scan.

In addition, C-STAR also used polarizing filters - similar to polarizing
sunglasses - at four different angles (vertical, horizontal, and left and
right diagonals) to help determine how rough and how foamy the ocean surface
is.

C-STAR flew four times on the ER-2 operating out of NASA's Ames Research
Center and flying along the coast of California in February. Data from the
flights were converted into images in which one can see images of Catalina
and other islands, and wind shadows on the lee sides of the islands.

When the color of the images is enhanced, Hood said, differences appear
that should yield wind direction information.

Technician Doug Huie of Mevatec, an SSL support contractor, inspects
C-STAR before it is mounted in its pallet (left) and after integration in
the belly of the ER-2 (right). The white object at left is an aerodynamic
shroud to protect the antenna.
Click on the images for the whole picture.

"The wind direction effect is very small," she said, "on
the order of a 1 to 2 K change in brightness temperature." C-STAR measures
an "apparent" microwave temperature, or brightness temperature,
on the Kelvin absolute temperature scale where a difference of 1 K equals
almost 2 deg. F.

Hood and others now must refine the data and analyze it in detail to
see if they proved their theory.

"We're pretty confident that we captured what we wanted to on this
series of flights," she said.

The data will be analyzed over the next few weeks, and an additional
flight is being considered to follow up on the results.